New Zealand is grappling with two consecutive extreme weather events—massive flooding followed by a cyclone—that have claimed at least 12 lives and left hundreds of thousands of people without power. The high winds and waters of Cyclone Gabrielle have washed away coastal roads on the north island and left bridges splintered and broken. Landslides have covered tarmac with slick mud, and houses and streets across have been left under feet of water, only weeks after heavy rain also caused widespread floods. The country has declared a national state of emergency for just the third time in its history.
New Zealand’s climate change minister, James Shaw, wasted no time in pointing the finger at the root cause of the weather disasters, telling the New Zealand parliament: “This is climate change.”
He may well be right, but the evidence from attribution studies is yet to come, says James Renwick, a climate scientist and professor at the Victoria University of Wellington. The cyclone itself isn’t unusual for New Zealand, as they regularly spin out of the tropics and get close enough to cause alarm, he says. “We’re in line for these things on a reasonably regular basis. Some of them are not that remarkable and some are absolutely catastrophic,” Renwick says.
But our warming planet may have increased the ferocity of this cyclone because of warmer ocean waters, says Olaf Morgenstern, an atmospheric scientist at New Zealand’s National Institute of Water and Atmospheric Research. Hotter oceans mean that if a cyclone hits, “it will be stronger, it’ll contain more moisture, more energy and sustain its energy for longer,” he says.
New Zealand has also experienced marine heat waves linked to La Niña, a cyclical Pacific weather system, which has dominated the region for the past three years. These may have given the tropical cyclone a boost. “Because it was anomalously warm, it didn’t lose that much intensity—it was still pretty strong when it got here,” Morgenstern says.
Record-breaking rainfall and flooding preceded the tropical cyclone and wreaked havoc on the north island in late January—this too seems likely to be connected to climate change. January broke a century-old record for Auckland’s wettest month, with 539 millimeters of rain recorded, half of that falling in a single day. That was truly unprecedented, Renwick says, but the likely impact of climate change on New Zealand will be more complex than simply more rain.
The biggest influence on the regional climate are the winds that blow over the country from west to east. These deposit huge volumes of rain on the west coast of the south island in particular. Milford Sound, the famous fjord there that’s popular with tourists, is one of the wettest places on Earth, receiving a mean annual rainfall of 6.8 meters. The island’s mountains then force moisture out of the air as it passes over them, casting a rain shadow that leaves the east coast relatively dry.
But introduce even subtle changes in the wind direction or the wind speed, and you can end up with big changes in local climate, Renwick says. Climate modeling suggests those westerly winds are likely to get stronger. “Whether or not they lie over New Zealand so much is a tricky one to answer, because there’s a few moving parts of that story, but the broad picture is slightly stronger winds through time,” he says. An increase in strength is expected to deliver more rain to the west coast, and less to the east, resulting in hotter temperatures.